The present invention relates generally to transmissions and, more particularly, to transmissions having countershaft brakes.
Heavy-duty vehicles often use non-synchronous manual transmissions that require a substantial level of operator skill. To avoid clashing of gears, it is necessary for an operator to control the speed of rotation of a drive gears on the drive shaft to match the speed of rotation of the collar that is supposed to engage with a particular one of the drive gears. The collars rotate with the drive shaft, while the drive gears are driven by countershaft gears which are driven by the engine through a clutch and an input shaft geared to the countershaft. During initial start-up when a stopped vehicle is to be shifted into first gear or reverse gear, and during an upshifting operation, it is generally necessary to slow or stop rotation of the countershaft so that rotation of the drive gears will be slowed or stopped. To facilitate shifting gears more quickly, clutch and countershaft brakes have been employed. These tend to involve substantial additional expense and inertia brakes are prone to wear.
It is desirable to provide an inexpensive countershaft brake. It is also desirable to provide a countershaft brake that is less prone to wear.
According to an aspect of the present invention, a transmission with a countershaft brake comprises a countershaft, a positive displacement pomp comprising a pump housing having a chamber with an inlet and an outlet, the pump being driven by the countershaft, and a flow restrictor disposed at or downstream of the chamber outlet, the flow restrictor being adjustable to increase and decrease an amount of flow restriction from the outlet of the chamber. Torque needed to drive the pump increases with an increasing flow restriction by the flow restrictor.
According to another aspect of the present invention, a method for braking a countershaft in a transmission comprises driving a positive displacement pump with the countershaft to pump liquid out of an outlet opening of the oil pump, and increasing resistance to driving of the pump.